By a well-known process AlCl.sub.3 is electrolyzed to produce aluminum metal and chlorine gas. However, the AlCl.sub.3 is frequently contaminated with compounds such as CO.sub.2, CO and SO.sub.2 and smaller amounts of organic pollutants such as hexachlorobenzene. Such organic pollutants diminish the value of chlorine gas formed in the process unless they are separated either prior to electrolysis or afterwards. When the chlorine generated by electrolysis is recycled for chlorination of alumina, the concentration of organic pollutants tends to build up over time and may become hazardous.
Gaseous impurities have heretofore been removed from AlCl.sub.3 by various processes that include a step of desubliming the AlCl.sub.3 into solid form. Such processes are disclosed, for example, in Schoener et al U.S. Pat. No. 3,930,800 issued Jan. 6, 1976 and King et al U.S. Pat. No. 3,956,455 issued May 11, 1976. Although such desublimation processes remove most impurities, the desublimed AlCl.sub.3 may still contain minor concentrations of organic pollutants such as hexachlorobenzene.
Foley et al U.S. Pat. No. 3,464,900 issued Sept. 2, 1969 discloses a process for production of aluminum from gaseous aluminum chloride wherein inert gases may be separated from the aluminum chloride. A fused salt electrolyte containing the chloride of a metal more electropositive than aluminum is electrolyzed to form a reductant that is dissolved in a molten cathode comprising an alloy of aluminum and the reductant. The molten alloy is then circulated to a reactor compartment wherein aluminum chloride is reacted with the reductant to form aluminum and a chloride of the reductant. Inert gas contaminants accompanying the aluminum chloride may be vented from the reactor compartment through a bleed tube.
In the process disclosed by Foley et al, gaseous aluminum chloride is reacted with a molten metal reductant rather than being dissolved in a molten salt bath. In addition, Foley et al do not electrolyze aluminum chloride but rather perform their electrolysis on a chloride of a metal that is more electropositive than aluminum, e.g. magnesium chloride. Magnesium formed by electrolysis of magnesium chloride is dissolved in aluminum and circulated as a molten aluminum-magnesium alloy to a reaction compartment wherein aluminum chloride is introduced.
It is a principal objective of the present invention to provide an economical method for removal of volatile impurities from AlCl.sub.3. A related object of a preferred embodiment is to produce aluminum metal from gaseous AlCl.sub.3 by electrolysis in a molten salt bath without an intermediate step wherein the gaseous AlCl.sub.3 is desublimed to solid form.
Additional objects and advantages of the invention will become apparent to persons skilled in the art from the following specification and claims.